The present invention relates to devices for measuring the velocity of a mobile target and, more particularly, to a system for measuring the angular velocity of a mobile target.
Doppler velocimeters rely on the phenomenon of Doppler shift to measure the radial velocity of a mobile target. A monochromatic transmitter beam is directed from the base station to the target. Part of this beam is scattered by the target in the direction of the base station. Because of the Doppler effect, the scattered signal received at the base station has its frequency shifted by an amount proportional to the target's radial velocity, which is that component of the velocity that is parallel to the displacement from the station to the target. The scattered signal is heterodyned (mixed) with the transmitted signal to produce a beat note whose frequency is the Doppler shift frequency. The radial velocity is then readily determined from the frequency of that beat note.
When tracking a mobile target, it is often important to know not just the target's radial velocity but also its angular or transverse velocity. The system discussed in the preceding paragraph merely measures the target's radial velocity and not this angular or transverse velocity component.
The shorter the wavelength of the transmitted beam, the greater the resolution and accuracy of the Doppler velocimeter. Therefore, optical techniques are preferable to conventional radio wave techniques. However, bulk optical systems must be maintained in critical alignment and they take up a great deal of space and weight.